George Olshevsky wrote:
>Heilmann, G., 1927. The Origin of Birds, D. Appleton & Co., New York: viii +
>210 pp.
>
>effectively squashed the idea that dinosaurs and birds were intimately
>related.
Well, it did until Ostrom came along with, as you recall, a dinosaur lineage
that Heilmann knew nothing about. I think it is fair to say that this
effectively unsquashed the dino-bird connection for many palaeontologists,
though not of course for Martin, Feduccia et al.
>BCF asserts that flight evolved in birds from the trees down. Some BADD
>paleontologists, and almost all paleornithologists, agree.
And, again, this has nothing to do with the question of which lineage birds
evolved from. Were they dinosaurs (arboreal or otherwise) or archosaurs
outside the dinosaur clade (arboreal or otherwise)?
>BCF asserts that theropod dinosaurs were the giant cursorial descendants of
>the arboreal archosaur lineage some of whose small, arboreal members
>eventually evolved into birds. With the discovery of the Madagascar "flying
>dromaeosaur" (elongate, stork-like fore and hind limbs; feather papillae on
>the ulna; killer claw on the second pedal digit) unveiled at Cathy Forster's
>talk at this year's SVP, it is virtually certain that Greg Paul's idea that
>dromaeosaurs were the flightless descendants of [pre-]archaeopterygids is
>correct.
Except that the term "pre-archaeopterygid" doesn't mean much as such (heck,
crossopterygian fishes fit that description!). The question here is, given
the use of the term "flightless descendant", whether Paul is right in
assuming that the common ancestor of dromaeosaurids and archaeoperygids was
flighted. I'd like to know more about this Madagascar critter - but my
understanding from previous discussions with George here is that he is not
claiming a flying ancestor for dromaeosaurs. As such it still strikes me
that "BCF" and "BADD" as George uses them are not really separable. The
real problem logically (and I think Feduccia errs here too) is the
assumption that if one supports a dromaeosaurid or broder dinosaurid
ancestry for birds one must favour the "ground-up" view of flight origins.
I think this is a straw man.
In fact there are not two, but (at least) six possibilities for a common
dromaeosaurid-archaeopterygid ancestor:
1. terrestrial dinosaur
2. arboreal dinosaur
3. terrestrial non-dinosaurid archosaur
4. arboreal non-dinosaurid archosaur
5. terrestrial bird (whatever a bird is)
6. arboreal bird
(to which we can add, as a subset of 5 or 6, whether the common ancestor was
flighted, primarily flightless or secondarily flightless - but surely we can
assume that that question only applies if 5 or 6 are true, unless we care to
postulate flying archosaurs that were neither birds nor, I suppose, pterosaurs)
Of these I think that 2 is the most likely (or appealing) to me, and 5/6 the
least likely (not to mention the definitional problem of defining the "bird"
clade that this formulation raises) - but conflating these hypotheses into
two, BCF and BADD, muddles the waters. George's term "dino-bird" begs the
question of whether the common ancestor of these animals would, if we had
it, be classifiable as a non-bird dinosaur, a non-bird archosaur outside the
dinosaur clade, or a bird (which would I suppose require that
dromaeosaurids, or any other theropod this hypothesis fits, be nested within
the clade that includes all taxa accepted as birds). As such, I find the
term meaningless beyond indicating that somewhere or other dromaeosaurids
and birds had a common archosaurian ancestor - which really says no more
than that (a) evolution is a fact and (b) birds and dinosaurs are both
archosaurs. And, of course, the terrestrial / arboreal difference has
nothing to do with taxonomy, though it does relate to selective forces - a
quite different issue IMHO.
>I fully expect that the central idea of BCF, that of small,
>acronomic archosaurs as common ancestors of dinosaurs and birds, as
>originally put forth by Abel and as modifed by the past couple of
>decades' research strengthening the dinosaur-bird connection, will
>become the dominant paradigm of dinosaur evolution.
But this again says nothing about whether these small, acronomic archosaurs
were dinosaurs or not - and surely that is the question du jour.
--
Ronald I. Orenstein Phone: (905) 820-7886 (home)
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